1. Field of the Invention
This invention relates to an improved catalyst and its use for the conversion of hydrocarbons, particularly for the isomerization of alkanes.
2. General Background
The isomerization of light naphtha is an increasingly important process for the upgrading of petroleum refiners' gasoline pool. The widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the need for "octane," or knock resistance, in the gasoline pool. In the early years of lead removal, refiners relied principally upon increasing the octane of products from catalytic reforming and fluid catalytic cracking operations. Refiners have largely capitalized on these relatively low-cost upgrading options. More attention has now focused on upgrading the relatively low-octane high-paraffin naphtha component.
There is a long history of catalyst and process technology for the isomerization of light alkanes. The recent expansion of interest, however, has led to significant improvements in this technology. Catalyst and process developments have led to lower operating temperatures, wherein product octane is favored by isomer equilibrium. Substantial reduction in the hydrogen requirement for a stable operation has resulted in a significant cost reduction, principally through elimination of the need for a hydrogen-recycle system. Both of the aforementioned developments have led toward a predominance of liquid in the isomerization reactor feed, in contrast to the vapor-phase operation of the prior art.
Catalysts exhibiting dual hydrogenation-dehydrogenation and cracking functions are applied widely in the petroleum refining and petrochemical industries to the reforming and isomerization of hydrocarbons. Such catalysts generally have the cracking function imparted by an inorganic oxide, zeolite, or halogen, with a platinum-group component usually imparting the hydrogenation-dehydrogenation function. A catalyst useful in isomerization should be formulated to balance its hydrogenation-dehydrogenation and cracking functions to achieve the desired conversion over a prolonged period of time, while effectively utilizing the expensive platinum group metal component.
The performance of a catalyst in isomerization service typically is measured by its activity, selectivity, and stability. Activity refers to the ability of a catalyst to isomerize the reactants into the desired product isomers at a specified set of reaction conditions. Selectivity refers to the proportion of converted feed reacted into the desired product. Stability refers to the rate of change of activity and selectivity during the life of the catalyst. The principal cause of low catalyst stability is the formation of coke, a high-molecular-weight, hydrogen-deficient, carbonaceous material on the catalytic surface. Workers in the isomerization art thus must address the problem of developing catalysts having high activity and stability, and which also either suppress the formation of coke or are not severely affected by the presence of coke.
Catalysts for paraffin isomerization containing a platinum-group metal component and a halide on an alumina support are known in the art. For example, U.S. Pat. No. 3,963,643 (Germanas et al.) teaches a method of manufacturing a catalyst useful in the isomerization of paraffins by compositing a platinum-group metal with gamma or eta alumina and reacting the composite with a Friedel-Crafts metal halide and a polyhalo compound. U.S. Pat. No. 5,607,891 (Travers et al.) teaches a catalyst consisting of chlorine, Group VIII metal, and a support consisting essentially of 85-95% eta alumina and the remainder gamma alumina and its use for benzene reduction and isomerization. However, the art does not suggest a catalyst having the particular characteristics of the present catalyst or the surprising benefits of using this catalyst in the context of modern, primarily liquid-phase, isomerization operations.
The art has attempted to identify characteristics of isomerization catalysts which are important to their performance. For instance, U.S. Pat. No. 4,032,434 issued to R. E. Rausch describes a catalyst in which the desired amounts of halogen is loosely correlated to the catalyst's surface area and tin content.
The catalysts normally comprise an inorganic oxide support, such as alumina. A single type of alumina rather than a mixture is the norm. However, U.S. Pat. No. 5,922,639 issued to F. Alario et al. presents a bifunctional hydrocarbon conversion catalyst, described as a naphtha reforming catalyst, which comprises a mixture of gamma and eta aluminas.